1. Field of the Invention
This invention pertains to an electrical touch switch having a resiliently flexible and depressible contactor sheet upon a relatively rigid printed circuit baseboard.
2. The Prior Art
The prior art touch switch has a relatively rigid baseboard with two or more circuit patterns which need to be electrically connected in order to provide continuity between the circuit patterns.
The contactor on the prior art touch switch has a substrate sheet of MYLAR, polycarbonate or polyester plastic of about 0.005 in. (0.13 mm) thick which is secured to the baseboard by some type of adhesive.
On the inner surface of the contactor sheet will be a conductive contactor dot which is normally held about 0.005-0.010 in. (0.13-0.25 mm) off of the circuit patterns by some type of spacer between the baseboard and the contactor. The conductive contactor dot typically is a painted-on area of a conductive paint having a powdered metal such as gold, silver, copper or aluminum for providing electrical conductivity. This conductive paint is usually applied by silk screening and resultant contactor dot is usually about 0.001 in. (0.025 mm) thick. The contactor dot typically is contiguous and has an area and a form precisely matching the area and form of the baseboard circuit patterns. The user of this touch switch places a finger against an outside surface of the contactor sheet and depresses the sheet and the contactor dot against the circuit patterns and upon contact of the contactor dot against both of the circuit patterns there is provided continuity between the patterns. When the switch user removes the finger, the resilience of the contactor sheet pulls the contactor dot off of the circuit patterns and continuity is broken.
Usually these touch switches involve a plurality of circuit patterns and there will be several individual switches in one assembled touch switch. Typical practice is to provide a single contactor dot for each pair of circuit patterns. A specific example of this type of contact switch would be for a calculator which would require separate individual switches for each of the numbers one through nine as well as more switches for the zero, decimal point and function selections.
The contactor for this type of switch requires that the substrate sheet be silk screened with conventional decorative paint to indicate whatever the purpose of a switch is to be, and then the substrate is again silk screened to apply the contactor dots. These contactor dots must be precisely registered with the indicia visible on the exterior side of the substrate. The entire contactor must then be precisely registered upon a baseboard so that each and every contactor dot precisely aligns with a respective pair of circuit patterns. If any contactor dot is not properly aligned or if any contactor dot is defective, the entire switch will be defective. It is inherent with this type of switch that a contactor having a given pattern of contactor dots is only useable on a corresponding baseboard and different contactors are required for each specific baseboard. Because of the registration problems inherent in the prior art, it is customary to make the contactor sheets one at a time.
The mylar substrate contactor has a maximum thermal tolerance range of -60.degree. F. to +330.degree. F. (-51.degree. C. to 165.degree. C.); the polycarbonate substrate has a maximum thermal tolerance range of -40.degree. F. to +275.degree. F. (-40.degree. C. to +135.degree. C.) and the polyester substrate has a maximum thermal tolerance range of -30.degree. F. to +250.degree. F. (-34.degree. C. to +121.degree. C.). Useage of touch switches has been restricted to environments in which these temperature ranges are not exceeded.
The foregoing substrate materials are also susceptible to penetration by objects such as pocket knives, pencil points and are also susceptible to being deformed by running a ball point pen over the substrate and atop of the contactor dot; this deforming is best envisioned as being considered a crease line. A crease line, as made by a pen or pencil, can deform the substrate enough so that the resiliency required to lift the contact dot off of the circuit patterns can be lost.
It has also been found that the corners and edges of the prior art contactor substrate sheets peel rather easily from the substrate board due to the pliable nature of the substrate.
It has also been found that the prior art contactor sheets tend to stress crack and break after use. The breaks almost always occur right across the contact dot. It is believed that one of the reasons for this phenomenon, which shortens the life expectancy of touch switches, is that the stiffest part of the contactor sheet is precisely where the contactor dot and the indicia are silk screened onto the substrate thereby increasing the thickness of the contactor sheet.
The most critical limiting factor in the useage of touch switches has been their limited current capacity due to the use of painted contactor dots. A further critical limitation has been the resistance across the painted contactor dots. Burning of the painted contactor dots has also been a problem.
The solvent resistance of the previous contactor sheet substrate has also been just more than sufficient.